There is growing recognition that surgical repair of the heart is most likely to be successful in infancy (when there is dramatic capacity for growth and remodeling). For a subset of patients, the optimal time for intervention is prior to birth. Current prenatal cardiac intervention (PCI) procedures clearly have technical limitations in both fetal visualization and instrumentation, taking up to 220 minutes, and often further surgery for the mother. Global Hypothesis: Integration of customized, computer-assisted, surgical visualization and navigation techniques into PCI will reduce procedural time and improve accuracy leading to improved fetal outcomes. The Phase II Specific Aims are: Specific Aim 1: Optimize software and hardware to exceed accuracy requirements for fetal probe positioning and tracking. Integration of new high-resolution ultrasound and tracking equipment will optimize system accuracy and image quality. The ultrasound probe calibration protocols will be made more robust, accurate, and ergonomic. Repeat phantom experiments will confirm accurate visualization and localization performance acceptable for human fetal application. Specific Aim 2: Optimize the computer-assisted PCI (CAPCI) user interface to maximally reduce procedure time. In Aim 2 the user interface will be optimized to increase ease of use, speed, accuracy, and image content during simulated PCI, overcoming the limitations of the Phase I prototypes. Specific Aim 3: Test the increased accuracy and efficiency of CAPCI in small and large animal trials. Following initial validation trials using in vitro phantom heart targets, the success of the CAPCI system will be determined by experiments that position a fetoscope and balloon catheter through the chest wall, left ventricular apex, and then across the aortic valve of an adult rat. Finally, further experiments will determine the success of CAPCI in catheter positioning across the aortic valve in the mid-gestation fetal sheep. These experiments will provide proof of principle to begin clinical trials. Significance: The development of computer-assisted methods to optimize PCI will increase the accuracy (and reduce the length of maternal anesthesia) of these procedures with a direct impact on procedural success. This technology has broad applicability to other ultrasound-guided procedures in the fetus, child, and adult.